When your rig is posting you'll see GRUB (GRand Unified Bootloader) loading. Hit escape, choose 'recovery console' and hit enter to boot. When you have your command line, type -minus quotes- "sudo nano /etc/X11/xorg.conf"
Your xorg.conf file will come up. ctrl+v scrolls down, ctrl+y scrolls up. Look for this section.
Identifier "nVidia Corporation G80 [GeForce 8800 GTS]"
Yours will probably say "nv" where mine says "nvidia". Using your arrow keys, backspace and all, change that to read "vesa". If it says "vesa" change it to "nv".
ctrl+o to save, hit enter to write, then ctrl+x to exit nano.
Back at the command prompt type "exit". this will bring you from the recovery console to your login, hopefully with a desktop. If it does, reboot, make sure X (your graphical desktop) comes up.
Post back with results and we'll take it from there.